Imaging device and camera system including sense circuits to make binary decision

ABSTRACT

An imaging device includes: a pixel array section having an array of pixels, each of which has a photoelectric converting device and outputs an electric signal according to an input photon; a sense circuit section having a plurality of sensor circuits each of which makes binary decision on whether there is a photon input to a pixel in a predetermined period upon reception of the electric signal therefrom; and a decision result IC section which integrates decision results from the sense circuits, pixel by pixel or for each group of pixels, multiple times to generate imaged data with a gradation, the decision result IC section including a count circuit which performs a count process to integrate the decision results from the sense circuits, and a memory for storing a counting result for each pixel from the count circuit, the sense circuits sharing the count circuit for integrating the decision results.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/470,516, filed Aug. 27, 2014, which is a continuation of U.S.application Ser. No. 13/930,827, filed Jun. 28, 2013, now U.S. Pat. No.8,842,206, issued Sep. 23, 2014, which is a continuation of U.S.application Ser. No. 12/846,285, filed Jul. 29, 2010, now U.S. Pat. No.8,488,034, issued Jul. 16, 2013, which claims the benefit of priorityfrom Japanese Patent Application Nos. 2009-197986, filed Aug. 28, 2009and 2010-092076, filed Apr. 13, 2010, the entire contents of each ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an imaging device like a CMOS imagesensor, and a camera system.

2. Description of the Related Art

Recently, CMOS image sensors have widely been used in digital stillcameras, camcoders, monitor cameras, etc., and the market for the CMOSimage sensors has expanded.

Each pixel in a CMOS image sensor converts input light to electronsusing a photodiode which is a photoelectric converting device, storesthe electron for a given period, and then outputs a signal reflectingthe amount of stored charges to an analog-digital (AD) converterincorporated in a chip. The AD converter digitizes the signal to be sentoutside.

The CMOS image sensor has such imaging pixels laid out in a matrix form.

FIG. 1 is a diagram showing the typical chip configuration of a CMOSimage sensor 10 which is a solid-state imaging device.

This CMOS image sensor 10 has a pixel array section 11, a row drivecircuit 12, an AD converter 13, a switch 14, an output circuit 15, a rowcontrol line 16, a vertical signal line 17, and a transfer line 18.

The pixel array section 11 has a plurality of pixels PX laid out in amatrix form in the row direction and in the column direction. Thevertical signal line 17 is shared by a plurality of pixels PX aligned inthe row direction, and is connected to the AD converter 13 arranged inassociation with each column.

The row drive circuit 12 selects only one of a plurality of rows, andenables the row control line 16 to read stored charges from the pixelsPX row by row.

The row control line 16 is formed by a single control line or aplurality of control lines to read stored charges from such pixels, orreset the pixels row by row.

Resetting herein means an operation of discharging stored charges fromthe pixels to set back the pixels to the state before exposure, and isexecuted as a shutter operation immediately after reading each row ofpixels or at the time of initiating exposure.

At the time of reading stored charges, analog signals transferred to theAD converter 13 via the vertical signal line are converted to digitalsignals, which are in turn sequentially transferred to the outputcircuit 15 via the switch 14 to be output to an image processingapparatus (not shown) located inside or outside the chip.

When reading one row of pixels is completed in the CMOS image sensor 10,a next row is selected, and similar charge reading, AD conversion, andsignal outputting are repeated. The completion of the processes on allthe rows completes the outputting of one frame of image data.

A hold circuit or a latch may be provided somewhere before the outputstage to pipeline the charge reading, AD conversion, and signaloutputting, but the CMOS image sensor is still unable to execute morethan one row of image data.

The time needed to finish processing every row of data defines the upperlimit of the frame rate of dynamic images.

JP-A-2002-44527 (Patent Document 1) and JP-A-2006-49361 (Patent Document2) have proposed an image sensor which has a laminate of pixels and ADconverters.

FIG. 2 is a conceptual diagram of a CMOS image sensor 10A which has alaminate of pixels and AD converters.

To help understand the concept, same reference numerals are given to thesame components as shown in FIG. 1.

The CMOS image sensor 10A in FIG. 2 has pixels PX and AD converters 13respectively arranged on different semiconductor substrates in an array.The two semiconductor substrates are laminated one on the other, witheach pixel connected to the respective AD converter by an analog signalline 17.

The use of such an architecture can ensure reading charges from multiplerows of pixels at a time, and parallel execution of AD conversion row byrow.

The data after conversion is temporarily transferred to a memory 19 tobe transferred to an image processing apparatus (not shown) locatedinside or outside the chip.

The adoption of such a laminate structure can dramatically improve theimaging speed at least in the imaging chip, thereby ensuring ultrafastframe imaging.

Further, development of a high-precision wafer adhering technique haslately attracted considerable attention. For example, JP-A-2007-234725(Patent Document 3) and JP-A-2006-191081 (Patent Document 4) describe atechnique of adhering a back-irradiation type image sensor and acircuit-mounted substrate opposite to each other, and transfer signalstherebetween via a metal pad.

This technique makes it possible to prepare a laminate structure asshown in FIG. 2 in the wafer-level fabrication, and connect pixels to ADconverters without implementing bump connection for each chip.

Since this technique allows individual chips to be cut out after thewafer-level fabrication, it is suitable for microprocessing and isconsiderably inexpensive.

JP-A-7-67043 (Patent Document 5) has proposed a new scheme of countingphotons in a time-divisional manner.

According to the counting scheme, binary decision on thepresence/absence of a photon input to a photodiode in a given period isrepeatedly performed multiple times, and the decision results areintegrated to acquire two-dimensional imaged data.

That is, signals from the photodiode in the given period are sensed, anda counter connected to each pixel is counted up by 1, regardless of thenumber of input photons when the number of photons input in that periodis equal to or greater than 1.

If the frequency of photon inputs is random along the time axis, theactual number of photons input and the count number are conform to thePoisson distribution, so that the numbers have a substantially linearrelation when the incident frequency is low, and can be corrected in anycase when the incident frequency is high.

Since the image sensor using such time-divisional photon counting treatsdata output from the pixels always as digital data, random noise orfixed nose originated from transmission and amplification of analogsignals do not occur.

At this time, it is only the photo shot noise and dark current generatedin the pixels that remain, and a very high S/N ratio can be acquiredparticularly in imaging with low illuminance.

SUMMARY OF THE INVENTION

The use of the structure in FIG. 2 can allow signals to be read out fromthe pixel array section fast in parallel and be subjected to ADconversion before being stored as data in the memory.

However, significant difficulties still remain in digitizing the dataand making the best use of the imaged data stored in the memory 19.

First, when a vast amount of data acquired at the frame rate several tentimes faster is transferred outside as it is, the transfer interface andthe chip for the subsequent image processing become very expensive. Ifthe frame rate is merely increased considerably over the sensing abilityof eyes, application of the image sensor is limited.

Therefore, it is desirable to take some new measures to add usefuleffects including an improvement of the image quality, if possible, inthe imaging chip and output data of a bandwidth which does not differmuch from that in the normal case by applying such ultrafast imaging.

However, Patent Document 2 hardly states data processing followingmemory storage.

In the literature cited in the description of the embodiment in PatentDocument 1, the fast reading performance is applied to achievement of“Sigma-Delta” based AD conversion.

However, this scheme makes it difficult to compensate for a variation incharacteristics of individual AD converters, and achievement of such ADconversion should not necessarily improve the image quality.

In general, the normal image sensor outputs an analog signal,photoelectrically converted by a pixel, and subjects the analog signalto AD conversion, so that various kinds of noise are mixed in theprocess of transmitting analog data and the process of converting theanalog data to digital data.

Configuration of a normal image sensor to have a laminate structureneeds analog signal connection between the substrates.

However, as compared with connection within the same substrate, theconnection between substrates is accompanied by a larger variation inimpedance, parasitic capacitance, etc., which may generate extra noise.

Patent Documents 5 and 6 have proposed imaging devices which use photoncounting.

Such an imaging device receives outputs from pixels directly in thedigital form, so that it is possible to completely eliminate randomnoise or fixed noise originated from analog signal processing which isinevitable in the normal image sensor. This leads to a potentially veryhigh S/N ratio.

Since photon counting needs extremely fast reading, however, the imagingdevices disclosed in those two patent documents have digital decisionfunctions provided in the individual pixels, and provided on the samesubstrate where the light receiving devices are disposed.

For example, a counter is needed for each pixel in Patent Document 5.

In Patent Document 6 which has achieved miniaturization of pixels, thepixels individually need 1-bit memories which are disposed planarlyalong with the light receiving devices.

In addition, the circuit which is called “1-bit memory” needs to alsohave a signal decision function, and needs more complex control and morecircuit elements than a simple latch.

This makes the number of apertures of pixels very small, so thatsufficient sensitivity cannot be obtained. In addition, a counter,located outside the pixel array though, is provided for each pixel.

According to the technique proposed in Patent Document 5, the number ofphotons that can actually be sensed is defined by the total number ofreadout decisions in one frame period to form a single image in imagingusing time-divisional photon counting.

When a 12-bit output is acquired in 4095 decisions on photon inputs, forexample, the actual number of sensible photons is equal to or less thanthe former number, and the square root of that number becomes photo shotnoise which occurs at random for each frame.

In case of imaging with low illuminance, the total number of photonsinput to a pixel in one frame period is, for example, 200 most of whichis actually counted without any problem. Therefore, the S/N ratio of thephoto shot noise becomes about the same as that of the analog sensor inthe related art, which makes the time-divisional photon countingadvantageous over the related art for it is free of analog transmissionnoise significantly larger than the photo shot noise.

In case of imaging with high illuminance, on the other hand, and analogsensor whose photodiode stores 10,000 electrons, for example, can countthat quantity of electrons at a maximum.

At this time, the photo shot noise is 100 e-rms, and the S/N ratiobecomes 100 times (40 dB) greater. The time-divisional photon countingcannot count about 1,600 electrons if a linear region is used inconsideration of the precision.

At this time, the photo shot noise is 40 e-rms, and the S/N ratioobtained is 40 times (32 dB) greater.

In case of a full digital imager which uses time-divisional photoncounting, therefore, the total number of counts needs to be increased inorder to improve the S/N ratio of imaging with high illuminance.

However, the total number of counts is restricted by the time of readingdata from the pixels at the time of making a decision on photon inputs.

While reading pixel data is the detection of a minute single photonsignal, random noise of the sensing circuit increases as the readingbecomes faster. Therefore, an increase in readout error ratio limits thedata readout time.

Suppose that data reading needs 400 nanoseconds. Normally, the readingoperation of an imager is destructive reading, so that a pixel inreading cannot store charges (charge storage being equivalent toexposure).

To secure the exposure time which is, for example, 90 percent of theframe period, therefore, the cycle time of decision which is the sum ofthe exposure time and the readout period needs to be 4 microseconds.

Provided that one frame period is 1/60 second, the then maximum numberof counts in decision reaches as high as 4,166. This number isinsufficient to secure a high S/N ratio at the time of high illuminance.

It is therefore desirable to provide an imaging device and a camerasystem which eliminate the need for handling analog signals to cancelout circuit noise originated from an AD converter and handling analogsignals, without reducing the number of apertures of pixels, therebyimproving the imaging performance at a low cost.

It is also desirable to provide an imaging device and a camera systemwhich optimize the setting of exposure when time-divisional photoncounting is used.

According to one embodiment of the invention, there is provided imagingdevice including a pixel array section having an array of pixels each ofwhich has a photoelectric converting device and outputs an electricsignal according to an input photon, a sense circuit section having aplurality of sensor circuits each of which makes binary decision onwhether there is a photon input to a pixel in a predetermined periodupon reception of the electric signal therefrom, and a decision resultIC section which integrates decision results from the sense circuits,pixel by pixel or for each group of pixels, multiple times to generateimaged data with a gradation, the decision result IC section including acount circuit which performs a count process to integrate the decisionresults from the sense circuits, and a memory for storing a countingresult for each pixel from the count circuit, the plurality of sensecircuits sharing the count circuit for integrating the decision results.

According to another embodiment of the invention, there is provided acamera system having an imaging device, an optical system which forms animage of a subject onto the imaging device, and a signal processingcircuit which processes an output image signal from the imaging device,the imaging device including a pixel array section having an array ofpixels each of which has a photoelectric converting device and outputsan electric signal according to an input photon, a sense circuit sectionhaving a plurality of sensor circuits each of which makes binarydecision on whether there is a photon input to a pixel in apredetermined period upon reception of the electric signal therefrom,and a decision result IC section which integrates decision results fromthe sense circuits, pixel by pixel or for each group of pixels, multipletimes to generate imaged data with a gradation, the decision result ICsection including a count circuit which performs a count process tointegrate the decision results from the sense circuits, and a memory forstoring a counting result for each pixel from the count circuit, theplurality of sense circuits sharing the count circuit for integratingthe decision results.

The embodiments of the invention can eliminate handling of analogsignals to cancel out circuit noise originated from an AD converter andhandling analog signals, without reducing the number of apertures ofpixels, thereby improving the imaging performance at a low cost.

It is also possible to optimize the setting of exposure whentime-divisional photon counting is used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the typical chip configuration of a CMOSimage sensor which is a solid-state imaging device;

FIG. 2 is a conceptual diagram of a CMOS image sensor which has alaminate of pixels and AD converters;

FIG. 3 is a diagram showing an example of the configuration of a CMOSimage sensor (imaging device) according to a first embodiment of thepresent invention;

FIG. 4 is a diagram showing one example of the circuit configuration ofa pixel according to the first embodiment;

FIG. 5 is a diagram illustrating a first example of access proceduresaccording to the first embodiment;

FIG. 6 is a diagram illustrating a second example of access proceduresaccording to the first embodiment;

FIGS. 7A to 7C are diagrams illustrating more concrete examples of theaccess procedures in FIG. 6;

FIG. 8 is a diagram showing an example of the configuration of a CMOSimage sensor (imaging device) according to a second embodiment of theinvention;

FIG. 9 is a diagram for explaining a cyclic access to pixel blocksaccording to the second embodiment;

FIG. 10 is a diagram showing the general image of a chip in according tothe second embodiment shown in FIG. 8;

FIG. 11 is a circuit diagram showing one example of a sense circuithaving a self-referring function;

FIGS. 12A to 12F present a timing chart for explaining an example of areading operation using the sense circuit with the self-referringfunction in FIG. 11 referring to the pixel in FIG. 4 by way of example;

FIG. 13 is a diagram showing an example of the configuration of thepixel block corresponding to the second embodiment using an internalamplified diode;

FIG. 14 is a diagram showing one example of the cross section of a CMOSimage sensor which employs a coupling-capacitance based connectionstructure via a capacitor;

FIG. 15 is a circuit diagram showing one example of a sense circuit witha self-referring function in the CMOS image sensor which employs thecoupling-capacitance based connection structure via a capacitor;

FIG. 16 is a diagram showing an example of the configuration of a CMOSimage sensor (imaging device) according to a third embodiment of theinvention;

FIG. 17 is a diagram illustrating the flow of an imaged data process athigh illuminance in the circuit in FIG. 16;

FIG. 18 is a diagram illustrating the flow of an imaged data process atlow illuminance in the circuit in FIG. 16;

FIGS. 19A to 19D are diagrams showing the concept of cycle switching inthe third embodiment;

FIG. 20 is a diagram showing an example where the dynamic range ofimaging is improved by carrying out counting cyclically with thecombination of a long cycle period and a short cycle period; and

FIG. 21 is a diagram showing one example of the configuration of acamera system to which a solid-state imaging device according to afourth embodiment of the invention is adapted.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowwith reference to the accompanying drawings.

The description will be given in the following order.

-   -   1. Outline of the Features of Imaging Device According to        Embodiment of the Invention    -   2. First Embodiment (First Configurational Example of Imaging        Device)    -   3. Second Embodiment (Second Configurational Example of Imaging        Apparatus)    -   4. Third Embodiment (Third Configurational Example of Imaging        Device)    -   5. Fourth Embodiment (Camera System)        <1. Outline of the Features of Imaging Device According to        Embodiment of the Invention>

From the viewpoint of fast parallel reading, an embodiment of theinvention realizes the optimal configuration of an imaging device (CMOSimage sensor) as a full digital image sensor using photon counting.

First, each pixel outputs an electric signal representing thepresence/absence of a photon input within a specific period. A sensecircuit receives the result of the presence/absence of a photon inputmultiple times in one frame period, and makes a binary decision on eachresult. The imaging device generates gradation data for each pixelthrough integration of the decision results.

Based on this basic configuration, the imaging device according to themode has the following characteristic structures.

The first characteristic structure is the laminate of pixels and sensecircuits using different semiconductor substrates. The pixels and thesense circuits are respectively formed in arrays, which are laminated torealize fast parallel reading without sacrificing the number ofapertures.

The second characteristic structure is the hierarchical arrangement of aplurality of sense circuits and a count circuit for integrating decisionresults, so that the sense circuits share the count circuit. The sharingof the count circuit with a plurality of sense circuits can ensureflexible optimization of the circuit scale and the processing speed.

The third characteristic structure is the function of adjusting theexposure time by changing the reset timing. The exposure time isadjusted by changing the reset timing, not the read timing, therebyrealizing flexible pipeline to the subsequent transfer process.

The fourth characteristic structure is the hierarchical arrangement ofthe pixels, the sense circuits, and the count circuit. The sharing ofthe sense circuit with a plurality of pixels, and cyclic access make itpossible to cope with smaller pixels while securing the exposure time.Further, the sharing of the count circuit with a plurality of sensecircuits can ensure flexible optimization of the circuit scale and theprocessing speed.

The fifth characteristic structure is the sensing using theself-referring function to enable the detection of one photon for apixel. The reset level and the signal level are read from a pixel, andthe two levels one of which is added with an offset are compared witheach other to carry out a binary decision. This cancels out apixel-by-pixel variation in reset level.

The mode which employs the above structures can provide the imagingdevice with the photon counting capability without sacrificing thenumber of apertures of pixels, and can completely eliminate random noiseand fixed noise which are originated from analog signal processing andwhich would not be normally inevitable in image sensors. At this time,it is only the photo shot noise and dark current for each pixel thatremain, thus achieving a very high S/N ratio, which ensure generation ofclear gradation images.

Since the sense circuits or the like can be arranged under the pixels,and a complicated analog circuit is not needed, the chip is mostlyoccupied by the pixel array alone, making it possible to contribute toreduction of the chip cost.

Further, the dynamic range can be significantly expanded withoutchanging the pixels by increasing the sampling number to form one frameor performing the sampling operation at different exposure timescombined.

Even if the pixels and the sense circuits are laminated using differentsubstrates, outputs from the pixels to the sense circuits need not havethe accuracy of analog outputs, so that the impedance of the signalwiring and a variation in parasitic capacitance does not affect asnoise.

Furthermore, digital reading using self-referring function considerablyimproves the decision accuracy.

The following elaborates a CMOS image sensor as an imaging deviceaccording to the mode which has the aforementioned features.

2. First Embodiment

FIG. 3 is a diagram showing an example of the configuration of a CMOSimage sensor (imaging device) 100 according to the first embodiment ofthe invention.

[Outline of General Configuration]

The CMOS image sensor 100 CMOS image sensor 100 has an pixel arraysection 110, a sense circuit section 120, a group of output signal lines130, a group of transfer lines 140, and a decision result IC section150.

The pixel array section 110 has a plurality of digital pixels DPX laidout in a matrix form in the row direction and in the column direction.

Each digital pixel DPX has a photoelectric converting device, and has afunction of outputting an electric signal according to an input photon.

The pixel array section 110 is formed on, for example, a firstsemiconductor substrate SUB1.

The sense circuit section 120 is formed on a second semiconductorsubstrate SUB2 different from the first semiconductor substrate SUB1.

The sense circuit section 120 has a plurality of sense circuits 121 laidout, for example, in a matrix form in the row direction and in thecolumn direction in one-to-one correspondence to the matrix of pixelsDPX in the pixel array section 110.

Each sense circuit 121 has a function of making a binary decision onwhether there is a photon input to the respective digital pixel DPX fora predetermined period upon reception of a signal therefrom.

The first semiconductor substrate SUB1 and the second semiconductorsubstrate SUB2 are laminated.

For example, the lamination is carried out in such a way that aplurality of pixels DPX formed on the first semiconductor substrate SUB1face a plurality of sense circuits 121 formed on the secondsemiconductor substrate SUB2 in one to one. The opposing pixel DPX andsense circuit 121 are connected by each output signal line 131 in theoutput signal line group 130.

In the example in FIG. 3, the output of a pixel DPX-00 located at the0th row and 0th column is connected to the input of a sense circuit121-00 located at the 0th row and 0th column by an output signal line131-00. The output of a pixel DPX-01 located at the 0th row and firstcolumn is connected to the input of a sense circuit 121-01 located atthe 0th row and first column by an output signal line 131-01.

The output of a pixel DPX-10 located at the first row and 0th column isconnected to the input of a sense circuit 121-10 located at the firstrow and 0th column by an output signal line 131-10. The output of apixel DPX-11 located at the first row and first column is connected tothe input of a sense circuit 121-11 located at the first row and firstcolumn by an output signal line 131-11.

The pixels and sense circuits located at the other rows and columns arelikewise connected, though not illustrated.

The outputs of the sense circuits 121 in the sense circuit section 120which are located at the same row are connected to a common transferline 141.

In the example in FIG. 3, the outputs of the sense circuits 121-00,121-01, . . . located at the 0th row are connected to a transfer line141-0.

The outputs of the sense circuits 121-10, 121-11, . . . located at thefirst row are connected to a transfer line 141-1. A similar connectionis made for the second and subsequent rows, through not illustrated.

The decision result IC section 150 has a function of integrating thedecision results from the sense circuits 121 multiple times pixel bypixel to generate two-dimensional imaged data with gradation.

The decision result IC section 150 has decision result ICs 151-0, 151-1. . . in association with the row arrangement of the sense circuits 121in the sense circuit section 120.

In other words, a decision result IC 151-0 is connected to the transferline 141-0 to which the sense circuits 121-00, 121-01, . . . located atthe 0th row are connected.

A decision result IC 151-1 is connected to the transfer line 141-1 towhich the sense circuits 121-10, 121-11, . . . located at the first roware connected.

The decision result IC 151-0 has a register 152-0 which holds a decisionvalue transferred along the transfer line 141-0, a count circuit 153-0which counts a value held in the register 152-0, and a memory 154-0which stores the counting result from the count circuit 153-0.

The decision result IC 151-1 has a register 152-1 which holds a decisionvalue transferred along the transfer line 141-1, a count circuit 153-1which counts a value held in the register 152-1, and a memory 154-1which stores the counting result from the count circuit 153-1.

According to the embodiment, the count circuit 153-0 of the decisionresult IC 151-0 is shared by a plurality of sense circuits 121-00,121-01, . . . .

Likewise, the count circuit 153-1 of the decision result IC 151-1 isshared by a plurality of sense circuits 121-10, 121-11, . . . .

[Function of Digital Pixel]

As mentioned above, the digital pixel (which hereinafter may be simplycalled “pixel”) DPX has the photoelectric converting device, and outputsan electric signal according to an input photon.

The CMOS image sensor 100 as an imaging device has a function ofresetting the pixels DPX and a function of reading signals therefrom,and can execute the reset function and the read function at arbitrarytimings.

The reset function resets the pixel DPX to a state where a photon is notinput. Each pixel DPX desirably has a lens and a color filter at itslight receiving surface.

Such basic functions of the pixel are similar to those of a normalpixel, except that the output of the pixel DPX does not need theaccuracy and linearity of an analog value.

One example of the configuration of the digital pixel will be described.

FIG. 4 is a diagram showing one example of the circuit configuration ofa pixel according to the embodiment.

FIG. 4 shows one example of a pixel circuit including three transistors.

A single pixel DPX pixel DPX has a photodiode 111, a transfer transistor112, a reset transistor 113, an amplifier transistor 114, a storage node115, and a floating diffusion (FD) node 116.

The gate electrode of the transfer transistor 112 is connected to atransfer line 117, and the gate electrode of the reset transistor 113 isconnected to a reset line 118.

The amplifier transistor 114 has a gate electrode connected to the FDnode 116, and a source electrode connected to the output signal line131.

In the unit pixel DPX, light input to the silicon substrate for thepixels generates a pair of an electron and a hole, the electron beingstored at the storage node 115 by the photodiode 111.

When the transfer transistor 112 is turned on at a given timing, thoseelectrons are transferred to the FD node 116 to drive the gate of theamplifier transistor 114.

As a result, a signal charge becomes a signal to the output signal line131 to be read out.

The output signal line 131 may be grounded via a constant current sourceor a resistor element to perform a source follower operation, or may betemporarily grounded before reading and then be rendered in a floatingstate to output a charge level set by the amplifier transistor 114.

The reset transistor 113 is turned on in parallel to and at the sametime as the transfer transistor 112 to pull out the electrons stored inthe photodiode 111 to the power supply, so that the pixel is reset tothe dark state before electron storage, i.e., to a state where a photonis not input.

Such circuit and operational mechanism of the pixels are similar tothose of an analog pixel, and, like those of the analog pixel, can havevarious kinds of variations.

While an analog pixel analogously outputs the total amount of inputphotons, however, a digital pixel digitally outputs the presence/absenceof the input of a single photon.

Therefore, the design concepts of an analog pixel and a digital pixeldiffer from each other.

First, a digital pixel needs to generate an electric signal large enoughfor the input of a single photon.

In the pixel circuit with an amplifier transistor as shown in FIG. 4,for example, it is desirable to make the parasitic capacitance at theinput node 116 of the amplifier transistor 114, which constitutes asource follower, as small as possible.

It is desirable to keep the amplitude of the output signal against theinput of a single photon sufficiently larger than random noise of theamplifier transistor 114.

Since the output signal of the digital pixel does not need the accuracy,the linearity and the operational range of the analog pixel, a lowvoltage similar to that needed for a digital circuit can be used for theinput/output power supply of the source follower. Further, thephotodiode may need a minimum charge storage capacity.

The CMOS image sensor 100 according to the embodiment is configured tohave the aforementioned first, second and third characteristicstructures as follows.

The CMOS image sensor 100 has the pixel array section 110 and the sensecircuit section 120 laminated using different semiconductor substrates.The CMOS image sensor 100 is configured in such a way that the pixelsand the sense circuits are respectively formed in arrays, which arelaminated to realize fast parallel reading without sacrificing thenumber of apertures.

The CMOS image sensor 100 is configured in such a way that a pluralityof sense circuits share a count circuit to ensure flexible optimizationof the circuit scale and the processing speed.

The CMOS image sensor 100 is configured in such a way as to have thefunction of adjusting the exposure time by changing the reset timing.The exposure time is adjusted by changing the reset timing, not the readtiming, thereby realizing flexible pipeline to the subsequent transferprocess.

Next, the outline of the general operation of the CMOS image sensor 100according to the first embodiment will be described.

For example, all the pixels DPX are reset at the same time, and signalsare read therefrom at a time after a given exposure time.

The presence/absence of a photon input to each pixel DPX within anexposure period is output as an electric signal to the output signalline 131, and is subjected to a binary decision in the correspondingsense circuit 121.

The sense circuit 121 sets “1” as a decision value when a photon isinput to the selected pixel, and sets “0” as a decision value when aphoton is not input to the selected pixel, and latches the decisionvalue.

That is, since the output signal from a pixel DPX is subjected to abinary decision as a digital signal according to the embodiment asdifferent from the normal configuration as shown in FIG. 2, an ADconverter is not involved here. Further, the decision speed issignificantly faster than that of the AD converter.

The decision value set and latched by the sense circuit 121 issequentially transferred to the register 152, arranged for each row,through the transfer line 141, and is subjected to a count process usingthe count circuit 153.

The transfer may be achieved by connecting the individual sense circuitsto the common bus sequentially by switches as shown in FIG. 1, or may beachieved by using a shift register.

In the count process performed by the count circuit 153, first, pixeldata read out previously is loaded into the count circuit 153 from thememory 154.

When “1” is stored in the register 152, “1” is added to the count value,and when “0” is stored in the register 152, the count value is notupdated.

Thereafter, the value of the count circuit 153 is written back into thememory 154, which completes the count process for one pixel.

This process is sequentially executed for one row of pixels. While sucha count process is carried out, next resetting and exposure areperformed on the pixels DPX.

Such digital reading is performed 1023 times in one frame period, forexample, and the total number of counts of input photons to each pixelDPX becomes 0 or greater, and 1023 or less.

Accordingly, 10-bit gradation data is generated pixel by pixel.

That is, the CMOS image sensor 100 operates as an arrayed photon counterwith a unique configuration.

As mentioned above, the individual pixels DPX are disposed on adifferent semiconductor substrate on a support circuit which includesthe associated sense circuits 121.

The pixels DPX and the sense circuits 121 are disposed in an array onthe respective semiconductor substrates. For example, the pixels DPX andthe sense circuits 121 are respectively formed on separate semiconductorwafers, which are in turn adhered together to achieve the laminate ofthe semiconductor substrates.

Further, it is desirable that at least some of the drive circuits forthe pixels DPX which are to be reset or subjected to data readout beformed on the same first semiconductor substrate SUB1 as the pixels DPXare formed.

This configuration can ensure fast pixel access and fast counting inparallel, so that the aforementioned multi-acquisition of data can becarried out in one frame period.

For example, the individual pixels perform resetting and reading at atime, and data transfer and counting are performed in the individualrows at a time.

[Access Procedures]

Next, the access procedures according to the embodiment will bedescribed.

FIG. 5 is a diagram illustrating a first example of access proceduresaccording to the first embodiment.

In FIG. 5, RST represents resetting, EXP represents exposure, and RDrepresents reading. In addition, TRF represents the transfer process,and CNT represents the count process.

In the example in FIG. 5, when one frame period is 1/30 second duringwhich reading is carried out 1023 times, for example, one cycle of theread RD is about 32 microseconds.

In this period, the reset RST and read RD are performed on the pixelsDPX, and the duration from the reset RST and the read RD is the periodof the exposure EXP.

A decision value which has been latched in the sense circuit 121 in theread RD is transferred to the register 152 to be counted, at which timethe exposure EXP, and the transfer pulse TRF and the count process CNTare carried out in pipeline.

That is, while the decision values which have been latched in the sensecircuits 121 in the cycle CYL1 are transferred in the row direction andare counted sequentially, the pixels are subjected to the reset RST inthe cycle CYL2, and the exposure EXP is initiated.

The CMOS image sensor 100 has an adjustment function of controlling theeffective exposure time to adjust the sensitivity by changing the resetRST while keeping the cycle period constant.

For example, although two or more photons may be input in the exposureperiod in imaging a bright subject, all of them are counted as a singlephoton, resulting in undercounting.

In such a case, the reset timing should be set closer to the read timingto shorten the exposure time, thereby dropping the sensitivity. This caneasily adjust the sensitivity during imaging without influencing othercircuit operations.

The imaging system averages the count values of all the effectivepixels, for example, and changes the reset timing of the imaging deviceto shorten the exposure time when the average count value exceeds agiven value. When the average count value is lower than the given value,on the other hand, the imaging system elongates the exposure time.

It is easy to install such a function, so that the optimal exposure timecan be automatically set by using the algorithm of binary search or thelike.

When there is a large number of pixels, horizontal transfer and thecount process need to be carried out at a high speed, which can howeverbe suppressed by performing the transfer of each row in multiplechannels using a plurality of counters.

Although it is desirable to form the count circuits 153 and the memories154 on the same substrate as the sense circuits 121 of the sense circuitsection 120 are formed, they may be disposed in lamination on a thirdsemiconductor substrate under the sense circuit section 120.

In consideration of power consumption and nose, for example, the pixelarray section 110 may be separated into a plurality of pixel blocks, sothat the pixel reading operation and the transfer operation for each roware carried out block by block.

Although the sampling is carried out 1023 times to generate 10-bitgradation in the foregoing embodiment, the dynamic range can be enlargedby increasing the number of sampling actions without changing thepixels.

When the sampling number is set to 16383, about 16 times theaforementioned number, for example, one cycle is 2 microseconds.

If this cycle period is fully used for exposure, the number of photonsin low illuminance mode can be counted in the same way as done normally,and the number of photons in high illuminance mode can also be countedaccurately up to 16 times the number of photons in normal mode. Thosenumbers are expressed as 14-bit gradation data.

Alternatively, the dynamic range may be improved efficiently byacquiring data with different types of exposure periods provided.

FIG. 6 is a diagram illustrating a second example of access proceduresaccording to the first embodiment.

FIG. 6 shows an example where the access procedures in FIG. 5 areevolved.

In this example, the reset timing is varied to provide two exposureperiods, first exposure EXP1 and second exposure EXP2, which arealternately repeated to acquire data.

The evolved use of such a technique ensures imaging in a wide dynamicrange in fewer sampling actions, making it possible to reduce the loadon the system.

FIGS. 7A to 7C are diagrams illustrating more concrete examples of theaccess procedures in FIG. 6.

In each of the concrete examples in FIGS. 7A to 7C, it is assumed thatthe first exposure EXP1 has an exposure time eight times that of thesecond exposure EXP2.

In the example in FIG. 7A, data acquisition in each of the firstexposure EXP1 and the second exposure EXP2 is carried out 511 times, anddata is individually counted and stored in two memories, first memoryMEM1 and second memory MEM2. 511 counts provide 9-bit gradation.

A pixel whose count in the first exposure EXP1 exceeds a given value isconsidered to have intense light input thereto, so that the count valuein the second exposure EXP2 is used.

In this case, the output is set to have 12-bit gradation, for example,and the pixel for which the count value in the second exposure EXP2 isused is shifted by three bits to be eight times larger as the output.

Alternatively, the output may be structured to have 9-bit gradation anda 1-bit flag indicating selection of exposure in order to reduce thenumber of output bits.

In the example in FIG. 7B, to increase the imaging sensitivity of a darksubject, the number of data acquisitions in long exposure is set greaterthan the number of data acquisitions in short exposure.

For example, a single data acquisition in the second exposure EXP2 isrepeatedly inserted for every four data acquisitions in the firstexposure EXP1, for example. Accordingly, data is acquired 1023 times inthe first exposure EXP1, and 255 times in the second exposure EXP2.

When the count in the second exposure EXP2 is used as the output, theoutput is shifted by five bits, for example, to be 32 times larger asthe output in consideration of the number of exposures.

At this time, the output can have 13-bit gradation at a maximum.Alternatively, the output may be structured to have 10-bit gradation anda 1-bit flag indicating selection of exposure.

In the example in FIG. 7C, to save the memory, test data is acquired 127times in the first exposure EXP1 first, and then data is acquired 512times alternately in the first exposure EXP1 and in the second exposureEXP2 each.

A pixel whose count in the first exposure EXP1 exceeds a given value inthe first 127 data acquisitions is considered to have intense lightinput thereto, so that a flag is set. When acquisition of the test datais completed, the count value in the memory is cleared once, except forthe flag. For the pixel with the flag set, data only in the secondexposure EXP2 is counted and stored in the memory thereafter.

For a pixel having no flag set, data only in the first exposure EXP1 iscounted and stored in the memory. The memory that is needed for thecounting operation per pixel is one 10-bit memory which has a flag inaddition to 9-bit gradation.

It is to be noted that when the first exposure EXP1 is selected, thegradation of the memory may be increased after test, instead of clearingthe memory.

Plural sets of exposure periods are provided by varying the resettiming, and data is read multiple times in each exposure period togenerate imaged data in the above manner, making it possible to executeimaging over a wide dynamic range that copes with a subject with a highcontrast which includes both a bright part and a dark part.

Although two types of exposure periods are used in the foregoingexample, three or more types of exposure periods may be used to providea variety of modifications to the synthesis algorithm.

It is desirable to synthesize imaged data with the number of inputphotons in a short exposure period being basically used for a pixel withhigh illuminance while the number of input photons in a long exposureperiod is generally used for a pixel with low illuminance.Alternatively, count values in plural types of exposures may be output,and data synthesis may be carried out at the time of image processingusing a DSP chip or the like located at a subsequent stage.

Although synthesis of imaged data with varied exposure times ispartially carried out by an existing image sensor, data acquisition attwo types of exposure times is carried out at an interval of one frametime, which brings about a problem such as a dynamic subject beingcolored with unnatural colors.

The present scheme of alternately executing acquisition of both datamultiple times in one frame period does not have such a problem.

More generally, it is desirable that data acquisition with cyclicexposure times should be carried out multiple times, and the acquisitionresults should be synthesized to generate image data.

3. Second Embodiment

FIG. 8 is a diagram showing an example of the configuration of a CMOSimage sensor (imaging device) according to a second embodiment of theinvention.

In the CMOS image sensor 100 according to the first embodiment, theindividual pixels DPX correspond one to one to the sense circuits 121.

However, the spaces required for the pixels DPX and the sense circuits121 should not necessarily be the same.

In addition, with the laminate of two substrates, the count circuits andmemories which are large in size may be laid out outside the pixel arrayregion in which fast and long distance transfer of data from each sensecircuit 121 is essential, and is likely to be restricted by the layout.

A CMOS image sensor 100A according to the second embodiment provides aflexible solution to the above problem by allowing a plurality of pixelsto share a single sense circuit.

In the CMOS image sensor 100A, a pixel array section 110A has aplurality of pixels DPX laid out in a matrix form in the row directionand in the column direction.

A plurality of pixels DPX in the same column and a selection circuitform a pixel block 160-0, 160-1, 160-2, 160-3, . . . .

The CMOS image sensor 100A includes a row drive circuit 170 and rowcontrol lines 180 for driving the pixels DPX in the pixel array section110A to output electric signals of the pixels DPX to output signal lines131.

The CMOS image sensor 100A includes a circuit block 200 which performsbinary decision on the electric signals transferred through the outputsignal lines 131, and integrates decision results multiple times, pixelby pixel, to generate two-dimensional imaged data with gradation.

The circuit block 200 has a sense circuit section 120A and a decisionresult IC section 150A provided therein.

The sense circuit section 120A has sense circuits 121-0, 121-1, 121-2,121-3, . . . laid out in correspondence to the pixel blocks 160-0,160-1, 160-2, 160-3, . . . .

The sense circuit 121-0 has its input connected to an output signal line131-0 to which the outputs of all the pixels DPX-00, DPX-10, . . . ,DPX-150 forming the pixel block 160-0 are commonly connected.

That is, the pixels DPX-00, DPX-10, . . . , DPX-150 share the singlesense circuit 121-0.

The sense circuit 121-1 has its input connected to an output signal line131-1 to which the outputs of all the pixels DPX-01, DPX-11, . . . ,DPX-151 forming the pixel block 160-1 are commonly connected.

That is, the pixels DPX-01, DPX-11, . . . , DPX-151 share the singlesense circuit 121-1.

The sense circuit 121-2 has its input connected to an output signal line131-2 to which the outputs of all the pixels DPX-02, DPX-12, . . . ,DPX-152 forming the pixel block 160-2 are commonly connected.

That is, the pixels DPX-02, DPX-12, . . . , DPX-152 share the singlesense circuit 121-2.

The sense circuit 121-3 has its input connected to an output signal line131-3 to which the outputs of all the pixels DPX-03, DPX-13, . . . ,DPX-153 forming the pixel block 160-3 are commonly connected.

That is, the pixels DPX-03, DPX-13, . . . , DPX-153 share the singlesense circuit 121-3.

For other pixel blocks (not shown), sense circuits are laid out in thesense circuit section 120A in such a way that each sense circuit isshared by a plurality of pixels.

The decision result IC section 150A has a capability of integratingdecision results from the sense circuits 121-0 to 121-3 multiple times,pixel by pixel, to generate two-dimensional imaged data with gradation.

The decision result IC section 150A has registers 152A-0 to 152A-3, aselection circuit 155, a count circuit 153A, and a memory 154A.

The registers 152A-0 to 152A-3 hold decision values in the respectivesense circuits 121-0 to 121-3 which have been transferred throughtransfer lines 141A-0 to 141A-3.

The selection circuit 155 sequentially selects the outputs of theregisters 152A-0 to 152A-3 to supply the decision values held in theregisters 152A-0 to 152A-3 to the count circuit 153A.

The count circuit 153A sequentially performs a count process on thedecision values from a plurality of pixels (four pixels in thisembodiment) whose row has been selected to be read out and which havebeen supplied via the selection circuit 155, and stores a countingresult for each pixel in the memory 154A.

Data on pixels previously read out is loaded into the count circuit 153Afrom the memory 154A.

The decision result IC section 150A according to the second embodimenthas the single count circuit 153A which is shared by a plurality of theregisters 152A-0 to 152A-3.

In other words, the CMOS image sensor 100A according to the secondembodiment allows a plurality of sense circuits 121A-0 to 121A-3 toshare the count circuit 153A.

The CMOS image sensor 100A according to the embodiment is configured toinclude the aforementioned fourth characteristic structure.

That is, the CMOS image sensor 100A is configured in such a way that aplurality of pixels share a sense circuit, and are cyclically accessedto ensure the exposure time and cope with smaller pixels.

Further, the CMOS image sensor 100A is configured in such a way that aplurality of sense circuits share a count circuit to achieve flexibleoptimization of the circuit scale and the processing speed.

Next, the outline of the general operation of the CMOS image sensor 100Aaccording to the second embodiment will be described.

As mentioned above, the pixel block 160 (160-0, 160-1, 160-2, 160-3, . .. ) is configured to include 16 digital pixels DPX and a selectioncircuit. The selection circuit selects one of the pixels, and resets orread data from the selected pixel.

In this embodiment, one pixel in the pixel block 160 is selectedaccording to the row control line 181 which is driven by the row drivecircuit 170.

At the time of reading, presence/absence of a photon input to theselected pixel is output to the output signal line 131 (131-0, 131-1,131-2, 131-3, . . . ) as an electric signal, which is in turn subjectedto binary decision in the sense circuit 121A (121A-0, 121A-1, 121A-2,121A-3, . . . ).

The sense circuit 121A (121A-0, 121A-1, 121A-2, 121A-3) sets “1” as adecision value, for example, when a photon is input to the selectedpixel, and sets “0” as a decision value when a photon is not input tothe selected pixel, and latches the decision value.

The decision values in the sense circuit 121A (121A-0, 121A-1, 121A-2,121A-3, . . . ) are first transferred to the register 152A (152A-0,152A-1, 152A-2, 152A-3).

The count circuit 153A is shared by four pixel blocks 160-0 to 160-3,and a count process is sequentially performed on data from the fourpixels in a row, selected and read, via the selection circuit 155.

Then, the counting result for each pixel is stored in the memory 154A.

That is, data of the previously read pixel is loaded into the countcircuit 153A from the memory 154A.

The count value is incremented by “1” when “1” is stored in the register152A (152A-0, 152A-1, 152A-2, 152A-3), and is not updated when “0” isstored therein.

Thereafter, the value of the count circuit 153A is written back into thememory 154A, which completes the count process for one pixel. Thisprocess is sequentially executed on the four pixels.

While such a count process is carried out, the pixel block 160 (160-0,160-1, 160-2, 160-3), and the sense circuit 121A (121A-0, 121A-1,121A-2, 121A-3) can make data reading and decision on the next row inparallel.

Such digital reading is carried out, for example, 1023 times in oneframe period to generate 10-bit gradation data for each pixel.

At this time, the count circuit 153A has a size of 10 bits, and thememory 154A has a capacity of 640 bits for each of 16×4 pixels has10-bit data.

That is, the CMOS image sensor 100A operates as an arrayed photoncounter having a unique configuration.

In case of this configuration, when the number of rows of the pixelarray is the number of rows in one block, and blocks are laid out onlyin the column direction, it is possible to form all the circuits on thesame semiconductor substrate.

When the imaging device has a lot of pixels, however, it is desirablethat the pixel blocks 160-0, 160-1, 160-2, 160-3 should be formed inlamination on different semiconductor substrates on a support circuitincluding the respective sense circuits 121A-0, 121A-1, 121A-2, 121A-3.

Then, it is desirable that the pixel array section 110A including thepixel blocks 160-0, 160-1, 160-2, 160-3, and the sense circuits 121A-0,121A-1, 121A-2, 121A-3 should be laid out in an array on differentsemiconductor substrates respectively.

In other words, it is desirable that the pixel array section 110Aincluding the pixel blocks 160-0, 160-1, 160-2, 160-3, and the sensecircuit section 120A including the sense circuits 121A-0, 121A-1,121A-2, 121A-3 should be laid out in an array on different semiconductorsubstrates respectively.

It is further desirable that the sense circuit sections 120A should beformed on a substrate as the circuit blocks 200 each including the countcircuit 153A and the memory 154A, and be laid out in an array.Alternatively, the memories 154A may be disposed in lamination on athird semiconductor substrate under the sense circuits.

Next, the cyclic access to pixel blocks according to the secondembodiment will be described below.

FIG. 9 is a diagram for explaining the cyclic access to pixel blocksaccording to the second embodiment.

It is assumed here that when all of the arrayed pixel blocks operateapproximately in parallel, accesses to the individual pixels arerepresented by an access to a single pixel block regardless of how manypixels are provided in the imaging device.

Sixteen pixels included in each pixel block 160 (160-0, 160-1, 160-2,160-3, . . . ) are accessed sequentially and cyclically.

Given that the frame rate is 1/30 second during which reading is carriedout 1023 times for each pixel, one cycle of the block process isapproximately 32 microseconds during which reading of 16 pixels needs tobe completed.

The time section along the horizontal axis in FIG. 9 represents time twhich is assigned to an access for each pixel in a block, and which hasa maximum width of 2 microseconds.

Since data reading from each pixel and a decision on the data are simpleoperations similar to reading from a semiconductor memory, the timewidth has a sufficient margin.

In the above cyclic access, reset RST and read RD of the individualpixels DPX are carried out cyclically.

In this case, although the access timing differs pixel by pixel, thesubstantial time of exposure EXP from the reset RST to the read RD isuniform for all the pixels.

The exposure time can be varied by changing the timing of the reset RSTwithin the cycle range, so that the sensitivity can be adjusted withoutinfluencing other circuit operations.

If the reset RST for each pixel DPX is set immediately after theprevious read RD (in the same time section to which the readingbelongs), for example, the exposure time becomes maximum to cope withimaging of a subject with low illuminance.

If the reset RST is set immediately prior to the read RD (in the timesection preceeding by one to the reading), on the other hand, theexposure time becomes minimum to cope with imaging of a subject withhigh illuminance. Alternatively, if the reset timing is set variable inseveral levels within the same time section, the exposure time can beselected more freely.

The count process CNT follows the read RD, and reading of a next pixelis initiated in parallel.

At time t4, for example, a pixel No. 4 is read, and a pixel No. 1 isreset. In parallel to the operation, a count process is performed on apixel No. 3.

Although reading of the pixel No. 4 and resetting of the pixel No. 1 areexecuted in serial in a time-divisional manner in this embodiment,reading and resetting of pixels provided with independent resetmechanism therein as shown in FIG. 4 can both be carried out in parallelat a time with two row control lines are driven.

According to the second embodiment, with different exposure times set bychanging the reset timing, it is possible to perform data acquisitionmultiple times, and generate imaged data using the data acquisition.

Specifically, the data acquisition needs to be performed according toschemes shown in FIGS. 7A to 7C which have been described in theforegoing description of the first embodiment, making it possible toimaging in a wide dynamic range which is compatible with imaging of asubject with high contrast.

As described above, the second embodiment has a hierarchical structurewhere a plurality of pixels DPX share the sense circuit 121A (121A-0,121A-1, 121A-2, 121A-3) and the register 152A (152A-0, 152A-1, 152A-2,152A-3), and a plurality of sense circuits 121A (121A-0, 121A-1, 121A-2,121A-3) share the count circuit 153A.

Sharing those circuits at what ratio is optimized by the relationbetween the aforementioned access time and the occupation spaces of theindividual circuits.

Since the access time of one pixel has a sufficient allowance in theembodiment, for example, a larger number of pixels may share a sensecircuit, or a larger number of sense circuits may share a count circuit.

FIG. 10 is a diagram showing the general image of a chip in according tothe second embodiment shown in FIG. 8.

A plurality of circuit blocks 200 are laid out in an array on asemiconductor substrate SUB2A in the example in FIG. 10.

The plurality of circuit blocks 200 are laid out in an array.

Formed on the semiconductor substrate SUB2A are a control circuit 210which controls a plurality of circuit blocks 200, a demultiplexer(DEMUX) 220 for demultiplexing the outputs of the circuit blocks 200,registers 230, a transfer line 240, and an output circuit 250.

In FIG. 10, output data from the pixel block 160 including a pluralityof pixels DPX and a selection circuit is subjected to a decision in thesense circuit 121A-0, and is then transferred to the register 152A.

A plurality of registers 152A-0 to 152A-3 share the count circuit 153Avia the selection circuit 155, and counting results are stored in thememory 154A which is, for example, a dynamic RAM (DRAM).

The circuit blocks 200 are laid out in an array on the semiconductorsubstrate SUB2A, and operate in parallel at a time to make a decision onthe data from the pixels selected in each circuit block 200 and countthe number of input photons.

The timing-based supply of data to the circuit blocks 200 and the rowdriving of the memories 154A are carried out together for the circuitblocks 200 laid out in the row direction by the control circuit 210disposed for each row.

The circuit blocks 200 are laid out in an array on a differentsemiconductor substrate SUB2B laminated on the semiconductor substrateSUB2A.

It is desirable that the circuit blocks 200, and a group of the pixelblocks 160 corresponding thereto should be disposed at the same pitches,so that the individual pixel blocks 160 and the respective sensecircuits are connected adequately.

When counting for one frame is completed, the counting results stored inthe memories 154A are latched in the registers 231 in the register group230 via the demultiplexer 220 one row of the pixel array at a time.

When data of all the rows forming a frame is output, the process for oneframe is completed.

This output mode is compatible with the output mode of the normalimaging device which sequentially outputs frame data row by row.

When all the pixels are driven to smoothly image a subject as a dynamicimage, it is desirable to have two memories 154A for the count andoutput operations, and to have memories for two frames of pixels in allthe pixels as a whole.

In this case, the two memories are used, switched alternately from oneto the other frame by frame, so that while one memory is used for thecount operation, the other one is used for the output operation.

Alternately, memories for one frame may be separated into two groups, anodd row and an even row, and the interline operation may be carried outto output data from the even row while pixels in the odd row are exposedand counted, and to output data from the odd row while pixels in theeven row are exposed and counted.

There may be a case where it is desirable to reduce the amount of datato be output according to the application of the imaging device. Forexample, imaged data from all the effective pixels is used for a staticimage, while it is frequently desirable to reduce the number of pixelsto reduce the amount of data for a dynamic image.

To cope with such a case, some imaging devices have a capability ofadding up data of a plurality of pixels, and outputting the data as datafor one pixel. Such addition is normally carried out with an adderseparately provided, which increases the circuit occupation spaceaccordingly.

According to the embodiment, by way of contrast, the storage location ina memory is shared by a plurality of pixels which shares a countcircuit, so that addition of pixel data can be carried out very easilyand flexibly.

According to the first embodiment shown in FIG. 3, for example, aplurality of pixels which share the count circuit 153 at least in therow direction are allowed to share the storage location in the memory154 to be able to add data from the pixels.

Alternatively, according to the second embodiment as shown in FIG. 8, aplurality of pixels which share the count circuit 153A are allowed toshare the storage location in the memory 154A to be able to effectflexible addition of pixel data both in the row direction and in thecolumn direction.

At the time of performing such addition, the amount of the memory in useis saved to become, for example, ¼ in the case of addition of data offour pixels.

When all the pixels are used for a static image and pixel data is addedfor a dynamic image, therefore, the entire memory 154A in FIG. 10 needsto be one frame of pixels in the entire pixels.

One frame is entirely used for a static image, while, for a dynamicimage, the memory is reduced in the addition and separated into twomemories, which are used, switched alternately from one to the otherframe by frame, for the count operation and the output operation.

Any of those operations can be effected merely by changing the addressto be selected at the time of accessing the memories, and can becontrolled easily.

According to the embodiment, counted data stored in the memories 154,154A are output directly. The data in those memories can be accessed atrandom pixel by pixel as data of a frame memory, so that an imageprocessing circuit, such as a DSP, may be further mounted on thesemiconductor substrate SUB2, SUB2A to perform image processing, such ascorrection of defects, de-mosaic operation and compression.

Further, addition of data of a plurality of pixels has an advantage thatwith a group of pixels whose data is to be added being regarded as asingle light receiving unit, the dynamic range of the output thereof canbe improved. When 10-bit counting is performed for each pixel, forexample, the output of adding data of four pixels will have 12 bits.

This addition can be performed flexibly according to the application;pixel data arranged in a two-dimensional array may be added for eachpixel group which shares a counter, and an adder may further be providedat the output stage to add data of pixel groups at the time ofoutputting the data.

Executing such step-by-step addition facilitates addition of all thepixels so that the pixels can be used as a single photon counter. Inthis case, the photon counter will have a huge dynamic range accordingto the number of pixels.

As mentioned above, each of the digital pixels to be used in theembodiment has a photoelectric converting device and a function ofoutputting an electric signal according to a photon input, and isconfigured as shown in FIG. 4, for example.

At the time of reading data from digital pixels, it is desirable toprovide the following self-referring function in sensing mode to cancelout a variation in output from one pixel to another.

That is, an output in a reset state and a signal output after exposureare read from a pixel, and a sense circuit compares both outputs witheach other with a given offset added to one of them to make a binarydecision.

FIG. 11 is a circuit diagram showing one example of a sense circuithaving a self-referring function.

A sense circuit 121B in FIG. 11 has switches SW121, SW122, SW123,capacitors C121, C122, inverters IV121, IV122, and a supply line L121for an offset signal OFFSET.

The switch SW121 has a terminal a connected to a first terminal of thecapacitor C121, and a first terminal of the capacitor C122, and aterminal b connected to a terminal SIG which is connected to an outputsignal line.

The second terminal of the capacitor C121 is connected to the inputterminal of the inverter IV121, a terminal a of the switch SW122, and aterminal a of the switch SW123.

The output terminal of the inverter IV121 is connected to the inputterminal of the inverter IV122 and a terminal b of the switch SW122.

The output terminal of the inverter IV122 is connected to a terminal bof the switch SW123 and an output terminal SAOUT.

An example of a reading operation using the sense circuit with theself-referring function as shown in FIG. 11 will be described referringto the pixel in FIG. 4 by way of example.

FIGS. 12A to 12F present a timing chart for explaining an example of areading operation using the sense circuit with the self-referringfunction in FIG. 11 referring to the pixel in FIG. 4 by way of example.

FIG. 12A shows a reset pulse RESET to be applied to the reset line 118in FIG. 4, and FIG. 12B shows a read pulse READ to be applied to thetransfer line 117 in FIG. 4.

FIG. 12C shows the ON/OFF state of the switch SW121, FIG. 12D shows theON/OFF state of the switch SW122, FIG. 12E shows the ON/OFF state of theswitch SW123, and FIG. 12F shows the offset signal OFFSET.

First, the switch SW121 and the switch SW122 are set on (ON) to applythe reset pulse RESET to the reset line 118, and read a pixel output inthe reset state onto the input terminal SIG.

Next, the switch SW122 is set off (OFF) to hold the reset output.

Then, the pulse READ is applied to the transfer line 117 for the pixelDPX to input a signal output representing the exposure result to theterminal SIG, thereby setting off the switch SW121.

During that period, the offset signal OFFSET input is kept 0 V.

Next, the level of the offset signal OFFSET is slightly increased to addan offset potential to the read signal via the capacitor C122.

As a result, the pixel output in the reset state and the output with aslight offset added to the read signal are compared with each other.

When a photon is input to the pixel in FIG. 4, the latter signal islower in potential than the former signal, so that “0” is output to theoutput terminal SAOUT.

When a photon is not input to the pixel, the opposite comparison resultis obtained, so that “1” is output to the output terminal SAOUT.

Finally, the switch SW123 is set on to latch the decision result.

This self-referring function can cancel out fixed noise for each pixeloriginating from a variation or the like in the threshold value of theamplifier transistor 114, and ensure an accurate binary decision even ona minute signal. Further, reset-originated kTC noise is also canceledout in the above sequence.

A similar effect can be expected even in correlated double sampling(CDS) in AD conversion of an analog signal.

It is to be noted that since the periods needed for two readings anddecisions are always constant in binary decision sensing, the influenceof thermal noise or flicker noise generated by the amplifier transistorof the pixel and the sense circuit themselves can be reduced as follows.

Since most of low-frequency band noise also appears (is superimposed) inboth readings, the influence can be cancelled out, sensitivity forhigh-frequency band noise can be restricted by the capacitive load ofthe sense circuit.

Therefore, the band of influential noise can be minimized by setting thecapacitive load as large as possible in an accurately sensible range.

In correlated double sampling in AD conversion, the period required forthe conversion often differs according to the level of the signal andthe number of bits, and is inevitably affected by the wide noise band.

The sense circuit is not limited to this example, and may be modified tocompare a reset signal added with an offset with a read signal to make adecision.

Alternatively, a read signal is acquired beforehand, after which a pixelis reset, a reset signal is then acquired, and the read signal and thereset signal, with an offset added to one of the signals, are comparedwith each other. In this case, although kTC noise cannot be canceledout, fixed noise or the like originating from a pixel-based variationcan be canceled out, so that the modification has an advantage that itis generally adaptable to any pixel configuration.

Even with the self-referring function installed, the sense circuit hasconsiderably fewer components than the normal AD converter, and does notneed large occupation space.

In case of realizing a digital pixel, it is an effective option to usean internal amplified type photodiode.

As an internal amplified type photodiode, for example, an avalanchephotodiode (APD) which accelerates a pair of a photoelectricallyconverted electron and a hole in electric field to cause avalancheamplification is known.

In this case, the pixel circuit as shown in FIG. 4 can be used, but thepixel does not need an amplifier transistor when the self-amplifiedphotodiode is used to acquire a sufficiently large signal.

FIG. 13 is a diagram showing an example of the configuration of thepixel block corresponding to the second embodiment using an internalamplified diode.

A pixel block 160C is formed by sets of only internal amplifiedphotodiodes 111C and transfer (selection) transistors 112C associatedtherewith.

That is, a pixel DPXC in this example is formed only by an internalamplified photodiode 111C and a transfer (selection) transistor 112Cassociated therewith. The gate electrodes of the transfer transistors112C of the individual pixels DPXC in the same row are connected to acommon transfer line 117C. The sources or drains of the transfertransistors of a plurality of pixels in each pixel block 160C areconnected to a common output signal line 131.

A reset transistor 113C is connected between each output signal line 131and a reset potential line LVRST. The gate electrodes of the individualreset transistors 113C are connected to a common reset line 118C.

In this example, each pixel DPXC is reset via the reset transistor 113C,the output signal line 131 and the transfer transistor 112C.

When the pixel blocks 160C are laminated on the sense circuit 121C, thereset transistors 113C may belong to the substrate of the pixel blocks160C, or may belong to the substrate of the sense circuits 121C.

In case of using adhered wafers as the laminate of semiconductorsubstrates, according to the manufacturing method of the related artdescribed earlier, signal connection between the pixels and the pixelblock, and between the sense circuits is assumed to be direct connectionvia a conductive pad electrode.

However, it is not easy to simultaneously expose a metal pad and aninsulating film which differ in polishing speed, and to simultaneouslypolish them to provide highly accurate flat surfaces which are neededfor adhesion and keep the adhesion strength.

In addition, the pad surface may be altered during polishing or beforeadhesion, thus causing improper insulation. When different chips areadhered, highly accurate direct connection via an electrode pad suffersa similar difficulty.

Meanwhile, transmission of digital data does not need high precision, sothat direct connection is not essential, and it is sufficient to makeconnection by means of coupling capacitance via a capacitor.

The capacitance of the capacitor is influenced by a productionalvariation originating from the size of the capacitor, the thickness of adielectric film, or the like, thus generating inherent noise whichdepends on the level of the signal for each capacitor. Therefore,transmission of analog signals brings about a lot of difficulties.

However, digital signals do not bring about such problems, and even asmall digital signal can be read out if combined with the aforementionedself-referring function.

FIG. 14 is a diagram showing one example of the cross section of a CMOSimage sensor 100D which employs a coupling-capacitance based connectionstructure via a capacitor.

In the example in FIG. 14, a digital pixel DPXE is formed on asemiconductor substrate SUB1E, so that electrons generated by aphotodiode 111E are transferred to an output electrode section 119 via atransfer transistor 112E.

A sense circuit 121E is formed on a semiconductor substrate SUB2E, andreceives an output signal from the pixels DPXE at an input electrodesection 122.

A capacitor CCP having a high dielectric film 300 sandwiched between itselectrodes is formed at a bonding surface BDS of both substrates SUB1Eand SUB2E. The output electrode section 119 of the pixel DPXE and theinput electrode section 122 of the sense circuit 121E are connectedtogether via the capacitor CCP.

After adhesion of the substrates, color filters 310 and microlenses 320are formed at the light receiving surfaces of the pixels DPXE.

The use of such a configuration can allow part of the self-referringsense circuit in FIG. 11 to be substituted by the coupling capacitor CCPto further simplify the circuit.

FIG. 15 is a circuit diagram showing one example of a sense circuit witha self-referring function in the CMOS image sensor which employs thecoupling-capacitance based connection structure via a capacitor.

Those components of the sense circuit in FIG. 15 which are the same asthe components in FIG. 11 are denoted by same reference numerals.

The sense circuit 121E in FIG. 15 is configured not to have the switchSW121 and the capacitor C121 of the sense circuit 121B in FIG. 11.

The digital pixel DPXE, as described above in conjunction with FIGS. 11and 12, outputs a reset level to the output electrode section 119 first.

The sense circuit 121E sets on the switch SW122, then sets it off tohold the reset level signal transferred via the coupling capacitor (CCP)in the input electrode section 122 which is the node set in a floatingstate.

That is, when the reset level is input to the output electrode section119, charges which cause the input electrode section 122 to reach thethreshold value of the inverter are stored in the input electrodesection 122 that serve as a storage node.

Thereafter, the digital pixel DPXE outputs a signal level to the outputelectrode section 119.

Further, the level of the offset signal OFFSET is shifted slightly tothe positive potential side to add a slight offset to the read signal.As a result, the inverters IV121, IV122 are driven to output a decisionresult to the output terminal SAOUT.

Finally, the switch SW123 is set on to latch the decision result.

In such a case, the coupling capacitor CCP can be interpreted as part ofthe sense circuit.

Signal transmission through coupling of the capacitor as describedreferring to FIGS. 14 and 15 can also be carried out according to thesecond embodiment where a plurality of pixels correspond to a singlesense circuit.

In the second embodiment, the output electrode section 119 extendingfrom the pixel is shared by a plurality of pixels in the pixel block.

4. Third Embodiment

FIG. 16 is a diagram showing an example of the configuration of a CMOSimage sensor (imaging device) according to a third embodiment of theinvention.

A CMOS image sensor 100B according to the third embodiment has afunction of repeatedly executing binary decision on presence/absence ofa photon input to a pixel in a predetermined exposure time, multipletimes in a unit frame period, and integrating decision results to derivethe amount of photons input to the light receiving section.

The CMOS image sensor 100B also has a function of variably setting thecycle period for decision within a plurality of cycle periods accordingto N times the unit cycle period (where N is an integer).

The CMOS image sensor 100B further has a mode of deriving the amount ofphotons input in the same unit frame period in fewer decisions in a longcycle period, and a mode of deriving the amount of photons input in manydecisions in a short cycle period.

The CMOS image sensor 100B further has a function of cyclicallyrepeating multiple decisions including a decision in a short cycleperiod and a decision in a long cycle period multiple times within theunit frame period, combining and integrating the decision results toderive the amount of photons input to the light receiving section.

In other words, the CMOS image sensor 100B has the optimal configurationto set the exposure of an imager using time-divisional photon counting.

That is, while it is desirable that the actual exposure time is longerto obtain a sufficient sensitivity in exposure with low illuminance,many decision counts are not needed.

To obtain a high S/N ratio with high illuminance, the total number ofcounts has a priority over the actual exposure time. For example, evenwhen 400 nanoseconds are used in the read operation as discussed above,a maximum of 16,666 counts in total can be secured if the cycle time fordecision is set to 1 microsecond.

At this time, the exposure time at most 60 percent of the frame periodcan be secured, which hardly matters in imaging with high illuminance.

At the time of imaging with low illuminance, on the other hand, thecycle time for decision has only to be set to four times or 4microseconds, for example, to secure the exposure time which is 90percent of the frame period.

The installation of the function of changing the cycle period fordecision to N times (N being an integer) basically does not need tochange the operational timing of the circuit, except for setting theexecution frequency of the read-and-decide operation to 1/N. Therefore,the control is easy, and the circuit scale is hardly increased.

Further, a set of a plurality of decisions at different cycle times maybe repeated within the unit frame period to make it possible to copewith imaging of a subject with high contrast, which includes a highillumination portion and a low illumination portion, and to secure asufficient exposure time for the low illumination portion.

According to the third embodiment, as apparent from the above, thenumber of decision counts can be increased at the time of imaging withhigh illuminance in the time-divisional photon counting to secure a highS/N ratio for photo shot noise, and to secure a sufficient exposure timefor the low illumination portion in addition.

Further, it is possible to cope with imaging of a subject with highcontrast, which includes a high illumination portion and a lowillumination portion, color shifting of a dynamic subject does notoccur, and a sufficient exposure time can be secured for the lowillumination portion. Furthermore, at the time of imaging with lowilluminance, power consumption can be reduced considerably.

The following will describe specific configurations and functions.

FIG. 16 is a diagram showing an example of the configuration of animaging device based on time-divisional photon counting.

A CMOS image sensor 100B includes a pixel array section 110B, a sensecircuit section 120B, registers (latches) 152B-0 to 152B-3, a countcircuit 153B, a memory 154B, and a selector 155B.

The registers 152B-0 to 152B-3, the count circuit 153B, the memory 154B,and the selector 155B constitute a decision result IC section 150B.

In the CMOS image sensor 100B, pixels are laminated on a circuitsubstrate in such a way that two pixels DPX1, DPX2 share a single sensecircuit 121B and the registers (latches) 152B-0 to 152B-3.

Further, four sense circuits 121B share the count circuit 153B and thememory 154B via the selector 155B.

Count data corresponding to the individual pixels are stored in thememory 154B at different addresses respectively.

FIG. 17 is a diagram illustrating the flow of an imaged data process athigh illuminance in the circuit in FIG. 16.

The process is carried out as follows in a unit cycle of 1 microsecond.

First, storage of a charge into the pixel DPX1 starts at time T0, andafter 600 nanoseconds, the sense circuit 121B starts reading of thepixel to perform binary decision.

At the end of the unit cycle, the decision data is stored in the latches152B-0 to 152B-3.

In the next cycle starting at time T1, storage of a charge into thepixel DPX1 starts again, and counting of the data stored in the latches152B-0 to 152B-3 starts.

Since the count circuit 153B is shared by four columns, latched data inpixels of the individual columns is sequentially sent to the countcircuit 153B via the selector 155B to be counted column by column.

In the count process for the pixel DPX1, first, corresponding count datafrom the memory 154B is set in the count circuit 153B, and the countvalue is counted up if the value latched in the latches 152B-0 to 152B-3are “1”, but is not updated if the latched values in the latches 152B-0to 152B-3 are “0”.

Thereafter, data in the count circuit 153B is written back at theoriginal address in the memory 154B, which completes the count processfor the pixel DPX1.

Alternatively, the above operation may be executed only when datalatched in the latches 152B-0 to 152B-3 is “1”, and no operations may beexecuted when the latched data is “0”.

Meanwhile, at the same timing of time T1, the sense circuit 121B startsreading data stored in the pixel DPX2.

As the process flow is repeated this way, the sense circuit 121B, thelatches 152B-0 to 152B-3, the count circuit 153B and the memory 154Bprocess, in pipeline, data from a plurality of pixels which share thecomponents.

Given that one frame period is 1/60 second at this time, it is possibleto make more than 16,300 counts equivalent to 14 bits, and data can beacquired with a high S/N ratio.

FIG. 18 is a diagram illustrating the flow of an imaged data process atlow illuminance in the circuit in FIG. 16.

The read process and the count process are skipped every one cycle fromthe process shown in FIG. 17, and data storage is maintained during thatperiod. That is, the length of the process cycle for each pixel isdoubled to be 2 microseconds.

At this time, the exposure time for each cycle is 1600 nanoseconds atmaximum, and an exposure time which is 80 percent of the frame periodcan be secured.

Given that one frame period is 1/60 second, the number of counts becomeabout a half of the counts in the process in FIG. 2. That is, the numberof counts stays becomes over 8,190 equivalent to 13 bits, which issufficient as the number of counts for low illuminance.

FIGS. 19A to 19D are diagrams showing the concept of cycle switching inthe third embodiment.

In FIGS. 19A to 19D, shaded portions indicate storage periods, andtransposition portions indicate read periods.

A cycle period N times the basic cycle (where N is an integer) can beset easily by skipping a read process and a count process accompanyingthe read process from the basic cycle.

The maximum number of counts at this time is approximately 1/N. As thecycle period is elongated as needed at the time of imaging with lowilluminance this way, it is possible to significantly reduce powerconsumption as well as secure the effective exposure period for a longtime.

The above cyclic switching and electronic shutter by resetting thepixels may be combined.

That is, although the storage periods in FIGS. 17 and 18 show themaximum storage periods, the substantial storage time can be adjustedfinely by resetting the pixels at an arbitrary timing during the storageperiod.

The combination of the cyclic switching and adjustment of the timing ofresetting the pixels can flexibly adjust the storage time, thus ensuringimaging under optimal exposure conditions.

In the actual imaging system, the system decides the brightness of asubject first as generally done in automatic exposure.

Then, high frequency sampling in a short cycle period is employed forimaging with high illuminance, and low frequency sampling in a longcycle period is employed for imaging with low illuminance.

In a simple example, imaging is started in a short cycle period first,and the mode is shifted to a low-illuminance imaging mode when theaverage number of photons to pixels in a screen in the unit frame periodis equal to or less than a given percentage of the total number ofcounts.

That is, the cycle period is increased along with reduction in the totalnumber of counts. Alternatively, imaging may be started in a long cycleperiod, and the mode may be shifted to a high-illuminance imaging mode.

FIG. 20 is a diagram showing an example where the dynamic range ofimaging is improved by carrying out counting cyclically with thecombination of a long cycle period and a short cycle period.

In this example, sampling is performed four times in a short cycle CYC1,and sampling is performed once in a cycle CYC2 which is four timeslonger than the cycle CYC1.

This sampling process is cyclically repeated to perform sampling, forexample, 4095 times in the cycle CYC1 and 1023 times in the cycle CYC2within one frame period. The count values for the individual pixels ineach cycle are independently stored in the memory at differentaddresses.

In sampling in the short cycle CYC1, the number of photons input to eachpixel during the total storage period in the sampling can be accuratelycounted at the time of high illuminance and low illuminance.

In sampling in the long cycle CYC2, on the other hand, the number ofinput photons is counted substantially accurately at the time of lowilluminance, but multiple count misses are included at the time of highilluminance.

The outputs are synthesized, for example, as follows pixel by pixel.

When the count value in the cycle CYC2 is equal to or greater than 256,this pixel is judged to be a high illuminance pixel, and, for example, avalue obtained by multiplying the count value in the cycle CYC1 by(total cycle time of CYC1 and CYC2/total storage time of CYC1) is usedas the output value of the pixel.

That is, the output is generated only from the count value in the cycleCYC1.

When the count value in the cycle CYC2 is less than 256, on the otherhand, this pixel is judged to be a low illuminance pixel.

As the output value of the pixel, for example, a value obtained bymultiplying the count value in the cycle CYC2 by (total cycle time ofCYC2/total storage time of CYC2) is used as the output value of thepixel is added with a value obtained by multiplying the count value inthe cycle CYC2 by (total cycle time of CYC2/total storage time of CYC2).The added value is then output.

That is, the count value in the cycle CYC1 and the count value in thecycle CYC2 are both used.

In this case, power consumption is over 60% of the power consumption inthe case of counting the number of photons only in the cycle CYC1, andthe number of photons input to a high illuminance pixel can be countedin the short cycle CYC1.

For a low illuminance pixel, a longer actual storage time can beobtained, and the sensitivity can be made higher accordingly.

Even when a high illuminance portion and a low illuminance portion existin the same screen, therefore, an optimal synthesizing scheme can beselected for each pixel, thus ensuring imaging with less nose and a widedynamic range.

Further, since sampling in two types of cycles is cyclically carried outmultiple times within one frame period, the results are averaged in eachcycle, and color shifting or the like originating from a difference inthe sample period from one pixel to another does not occur even inimaging a moving subject.

In case where high sensitivity with low illuminance is preferred, forexample, the number of samplings in the cycle CYC2 may be increased, andthe number of samplings in the cycle CYC1 may be decreased accordingly.In this case, the actual storage time becomes longer.

If the actual storage time is sufficiently long, the output from a lowilluminance pixel may be generated only from the count value in thecycle CYC2. At this time, the output from a high illuminance pixel isgenerated only from the count value in the cycle CYC1.

In addition, imaging may be carried out with three or more types ofcycles combined. There are various variations in the scheme ofsynthesizing the output from the count values in different cycleperiods.

The solid-state imaging devices according to the foregoing first, secondand third embodiments can be applied as an imaging device for a digitalcamera and a video camera.

5. Fourth Embodiment

FIG. 21 is a diagram showing one example of the configuration of acamera system to which a solid-state imaging device according to afourth embodiment of the invention is adapted.

As shown in FIG. 21, a camera system 400 has an imaging device 410 towhich the CMOS image sensor (solid-state imaging device) 100, 100Aaccording to the embodiment is adaptable.

The camera system 400 includes an optical system for guiding input lightto the pixel region of the imaging device 410 (forms the image of asubject), for example, a lens 420 for forming the image of input light(imaging light) on the imaging surface.

The camera system 400 further includes a drive circuit (DRV) 430 whichdrives the imaging device 410, and a signal processing circuit (PRC) 440which processes the output signal of the imaging device 410.

The drive circuit 430 has a timing generator (not shown) to generatevarious timing signals including a start pulse and a clock pulse todrive internal circuits of the imaging device 410, and drives theimaging device 410 in response to a predetermined timing signal.

The signal processing circuit 440 performs predetermined signalprocessing on the output signal of the imaging device 410.

The image signal processed by the signal processing circuit 440 isrecorded on a recording medium, such as a memory. The image informationrecorded on the recording medium is hard-copied by a printer or thelike. The image signal processed by the signal processing circuit 440 isdisplayed as a dynamic image on a monitor formed by a liquid crystaldisplay or the like.

As described above, the installation of the foregoing solid-stateimaging device 100, 100A as the imaging device 410 in the imaging devicefor a digital camera or the like can realize a camera with low powerconsumption and high precision.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. An imaging device comprising: an array of pixelseach of which has a photoelectric converting device and outputs anelectric signal according to an input photon; reset circuitry configuredto reset each of the pixels to a reset state where a photon is notinput; a plurality of sense circuits each of which makes a binarydecision on whether there is a photon input to a pixel in apredetermined period upon reception of the electric signal therefrom,the binary decision for a respective pixel being made by reading asignal in the reset state, reading a signal after exposure to an inputphoton, adding an offset to one of the signal in the reset state or thesignal after exposure, and comparing the signal in the reset state andthe signal after exposure, one of which is added with the offset; anddecision result circuitry configured to integrate decision results fromthe sense circuits, pixel by pixel or for each group of pixels, whereinthe decision result circuitry includes a count circuit configured toperform a count process to integrate the decision results from the sensecircuits, and a memory for storing a counting result for each pixel fromthe count circuit.
 2. The imaging device according to claim 1, whereinthe plurality of pixels and the plurality of sense circuits are formedin one-to-one correspondence, and connected together respectively, andevery predetermined number of sense circuits in the plurality of sensecircuits share the count circuit.
 3. The imaging device according toclaim 2, wherein the plurality of pixels are laid out in a matrix formin a row direction and in a column direction, the plurality of sensecircuits are laid out in a matrix form in a row direction and in acolumn direction, and connected in one-to-one correspondence to theplurality of pixels, and those sense circuits in the plurality of sensecircuits which are located in a same row or a same column share thecount circuit.
 4. The imaging device according to claim 1, wherein aplurality of pixels share the count circuit.
 5. The imaging deviceaccording to claim 4, further comprising a plurality of pixel blockseach including a plurality of pixels and selection circuitry configuredto select from the included plurality of pixels, wherein independentsense circuits of the plurality of sense circuits are arranged inassociation with the pixel blocks.
 6. The imaging device according toclaim 5, wherein the selection circuitry is configured to cyclicallyselect each pixel in the pixel block, and output a signal of theselected pixel to a respective sense circuit, and the respective sensecircuit makes a decision on whether a photon is input to each pixel in agiven period from previous selection to current selection.
 7. Theimaging device according to claim 6, wherein the reset circuitry isconfigured to adjust an exposure period by inserting a reset process toset the exposure period constant for the individual pixels betweenselective outputting of each pixel and next selective outputting in thepixel block.
 8. The imaging device according to claim 1, wherein thedecision result circuitry is configured to add count values for aplurality of pixels via the count circuit shared by the pixels.
 9. Theimaging device according to claim 1, wherein the plurality of sensecircuits are configured to repeatedly execute binary decisions in a unitframe period multiple times, a cycle period of the decisions beingvariably set within a range of a plurality of cycle periods according toN times a unit cycle period (N being an integer), and the decisionresult circuitry is configured to integrate the decision results toderive an amount of photons input to a light receiving section of thearray of pixels.
 10. The imaging device according to claim 9, whereinthe decision result circuitry is configured to one of carry outderivation of the amount of photons input in a same unit frame period ina small number of decisions over a long cycle period, or carry out thederivation of the amount of photons input in a large number of decisionsover a short cycle period.
 11. The imaging device according to claim 10,wherein a plurality of decisions including a decision in a short cycleperiod and a decision in a long cycle period are further cyclically madein the unit frame period, results of the decisions are combined andintegrated to derive an amount of photons input to the light receivingsection.
 12. The imaging device according to claim 9, wherein a smallnumber of decisions are made over a long cycle period in imaging at lowilluminance, and a large number of decisions are made over a short cycleperiod in imaging at high illuminance.
 13. The imaging device accordingto claim 1, wherein the offset is added to the signal after exposure.14. The imaging device according to claim 1, wherein the offset is addedto the signal in the reset state.
 15. An imaging device comprising: apixel array section having an array of pixels each of which has aphotoelectric converting device and outputs an electric signal accordingto an input photon; a reset circuit section configured to reset each ofthe pixels to a reset state where a photon is not input; a sense circuitsection having a plurality of sense circuits each of which makes abinary decision on whether there is a photon input to a pixel in apredetermined period upon reception of the electric signal therefrom,the binary decision for a respective pixel being made by reading asignal in the reset state, reading a signal after exposure to an inputphoton, adding an offset to one of the signal in the reset state or thesignal after exposure, and comparing the signal in the reset state andthe signal after exposure, one of which is added with the offset; and adecision result IC section which integrates decision results from thesense circuits, pixel by pixel or for each group of pixels, wherein thedecision result IC section includes a count circuit configured toperform a count process to integrate the decision results from the sensecircuits, and a memory for storing a counting result for each pixel fromthe count circuit.
 16. An imaging method, performed by circuitry,comprising: resetting, by reset circuitry, a pixel, of an array ofpixels of an imaging device, to a reset state where a photon is notinput; reading, by sense circuitry, an electric signal from aphotoelectric converting device of the pixel when the pixel is in thereset state; reading, by the sense circuitry, an electric signal fromthe photoelectric converting device of the pixel when the pixel isexposed to an input photon; adding, by the sense circuitry, an offset toone of the electric signal when the pixel is in the reset state or theelectric signal when the pixel is exposed to the input photon; making,by the sense circuitry, a binary decision on whether a photon is inputto the pixel in a predetermined period upon reception of an electricsignal therefrom, the making the binary decision including comparing thesignal in the reset state and the signal after exposure, one of which isadded with the offset; integrating, by decision result circuitry, thebinary decision results, pixel by pixel or for each group of pixels; andstoring, in memory, a counting result for each pixel.